You can't just slather yourself with sunscreen in the morning, head out for a long day at the beach and expect to come home sunburn-free. If you do take this approach, you'll probably end up with skin that's painfully roasted -- not to mention the serious conditions, like skin cancer, you'll have put yourself at risk for. Avoiding these risks, however, is easy. All you have to do is pay attention and reapply your sunscreen throughout the day. The challenge is in knowing how often to do it.

There are actually quite a few factors you need to take into consideration when planning your sunscreen coverage, including the sunscreen's sun protection factor (SPF), the time of day you'll be out in the sun and the kind of activity you have planned. SPF numbers in particular -- which you can't miss on the front of sunscreen bottles -- can help provide a basic estimate for how often you should reapply your sunscreen. Here's how it works: If you are using a sunscreen with an SPF of 15, and it usually takes you 15 minutes to get a sunburn, all you have to do is multiply the two numbers together to figure out about how long your sunscreen should last [source: WebMD]. In this case, it would be 225 minutes, or 3 hours and 45 minutes.

While that seems easy enough, the formula doesn't necessarily hold true when you're dealing with SPF numbers that are higher than 30. And, unfortunately, there are numerous other factors --- including humidity, sweating and contact with water -- that can change that number by decreasing the overall effectiveness of sunscreen [source: Mayo Clinic]. So if you're planning on going swimming or doing any other sort of strenuous activity, you'll need to reapply often for your sunblock to be of much use.

Another thing that influences the power of sunscreen when you're outside is the time of day. Between the hours of 10 a.m. and 4 p.m., UV rays are stronger, even on cloudy days. Most experts recommend you try to avoid the outdoors all together during these times. But if you absolutely must be outside, reapply your sunscreen as often as you can -- every couple of hours, or even every hour, if possible [source: Mayo Clinic].

Read on to get lots more information on sunscreen and why it's so important to protect yourself when you're outdoors.